The present invention relates to a method of slicing a silicon ingot with a wire saw which can freely adjust a bow to be allotted to a wafer and is also concerned with an apparatus therefor.
A wafer useful as a substrate for semiconductor devices is sliced off an ingot. Such an ingot is prepared by a CZ crystal growing method or the like. After top and tail parts are cut off the ingot, the ingot is processed by a crystal surface grinding machine and then sliced to a plurality of wafers having predetermined thickness. A commonly used slicer is an inner diameter saw, but a wire saw using a high tension steel wire or the like has been recently used to cope with enlargement of wafers in size.
A conventional wire saw has a wire wound over a plurality of grooved rollers with a predetermined pitch. The wire is uncoiled from a reel, carried over the grooved rollers along one direction, and coiled onto another reel. The wire may be reciprocatively carried between two reels. An ingot is pressed onto the wire, while an abrasive slurry is supplied to the wire. Due to the cutting force of the wire, the ingot is sliced to a plurality of wafers having a thickness approximately equal to the pitch with which the wire is wound over the grooved rollers.
After a sliced wafer is lapped, chemically etched and then polished, it is processed in device-making steps such as CVD or epitaxy. The wafer is likely bowed during the processing. A bow shall be controlled within a predetermined range; otherwise it would be difficult to build up a semiconductor having predetermined structure with high accuracy.
In order to properly control a final bow allotted to a wafer, it is preferable to slice an ingot to wafers having a shape which compensates for deformation during the device-making steps. A properly controlled bow effectively cancels deformation of wafers during CVD, epitaxy, etc. and enables construction of semiconductor devices with accurate design.
For instance, Japanese Patent Application Laid-Open 8-323741 discloses use of the displacement of grooved rollers for slicing an ingot to properly bowed wafers. A bearing part for grooved rollers is thermally expanded by use of friction heat during rotation of grooved rollers or by temperature control so as to displace the grooved rollers along an axial direction.
However, the proposed method requires precise temperature control, since the axial displacement of the grooved rollers depends on the thermal expansion of the bearing part. In addition, since a pair of grooved rollers are necessarily shifted in the same displacement distance along the axial direction, the grooved rollers shall be located and preset with high dimensional accuracy. Such accurate setting needs a fairly long time. Consequently, the apparatus lacks in operatability, and properly bowed wafers are obtained at a poor yield ratio.